Fluid-based medicament ejectors are used to dispense medicament as an aerosol. An example of such an ejector is the metered dose inhaler. Metered dose inhalers provide a much-needed drug-delivery method that allows patients to aspirate medication rather than swallow a pill, or drink or inject medication. In some cases, as with medications that directly target the patient's lungs, aspiration enables the medicine to reach the target area more quickly. In addition, aspiration is typically considered to be less painful than other drug-delivery methods.
As with other methods of taking medications, it is desired that the dosage levels be determinable and consistent. With solid or liquid medicaments, dosage level may be fairly accurately established. However, aerosol-based medicaments may be more difficult to administer accurately. For one reason, a mechanical device typically is relied upon to generate a dosage. It is also more difficult for a patient to receive a dosage effectively. For instance, with an inhaler, the patient must inhale an aerosol spray containing the medicament. The effectiveness of the dosage may depend, for example, on how well the patient inhales, and the orientation and position of the inhaler relative to the patient's mouth.
Effective delivery of aerosol medicaments also may depend on the consistent functioning of the inhaler. In particular, the inhaler should produce a desired quantity of medicament during a period of time that allows the aerosol medicament to be inhaled by the patient. The quantity of medicament is not the only issue with aerosols. It has been shown that the velocity and aerodynamic particle size of the droplet influence the location of aerosol deposition in the lung. Particles that are larger than approximately 6 microns tend to deposit in the mouth and trachea. Particles that are between approximately 2-6 microns tend to deposit in the bronchi and bronchioles. Particles that are between approximately 0.5-2 microns tend to deposit in the terminal bronchioles and alveoli. The deposition of particles below approximately 0.5 microns is minimal, these particles are more susceptible to being expelled upon exhalation. For many drugs, deposition in the lower airways is most desirable, and hence require aerodynamic particle sizes in the range of approximately 1-5 microns. Metered dose inhalers may produce a large range of droplet sizes within a single puff, including droplets both above and below the ideal range. Those droplets that are too small are not retained by the lungs, and are instead exhaled out of the body. Likewise, those droplets that are too large are not absorbed by the lungs, but instead deposit in the extrathoracic and upper airways. They are subsequently swallowed and absorbed in the gastrointestinal tract. As a result, in order to obtain dependable and consistent results with aerosol-based medicament ejectors, it is useful to maintain consistent quantity and quality of dosages.